A current-mode logarithmic amplifier (log amp) receives current inputs at a node held close to a defined potential and processes them so as to generate an output voltage that is proportional to the decibel-scaled magnitude of such currents. To help in differentiating this type of log amp from other types, such as RF log amps based on entirely different principles, this type is identified as an Instrumentation Log Amp (ILA). These are usually, though not necessarily, fabricated in monolithic form.
In common application of an ILA, the current in a photodiode coupled to an optical data fiber is accepted and this generates a logarithmically-scaled output, indicating the optical power in the fiber. In general, the photodiode current may be as small as 10 pA (10−11 A) and as large as 10 mA (10−2 A), corresponding to a decibel range of 20 log10(109)=180 dB. This is why logarithmic compression is an essential and integral aspect of the function of a high-performance ILA.
There is a need for ILAs that provide more complex operations, one of which is the management of a large number of input channels. That is, monolithic solutions having the capability of multiplexing a large number of input channels are needed, while keeping down the cost and the required space on a printed-circuit board, relative to using a single ILA per channel.
FIG. 1 shows a prior art embodiment of an array of multiplexed instrumentation log amplifiers (ILAs) 10. A set of current source inputs such as 12 provide input signals to an array of instrumentation logarithmic amplifiers such as 14. An array 18 of switches such as 16 multiplex the outputs of the ILAs, meaning that one of the ILA outputs is selected as the input to the analog-to-digital converter 20. This solution does not integrate the ILAs and the multiplexer and does not perform as well as one would like.
However, because these current-mode signals can be so very small, attempts to multiplex the inputs using MOS transistors as switches are fraught with difficulties due to the large relative magnitude of the switching transistors' displacement charges as each channel is selected. These charges are invariably unequal in their positive and negative directions and average out to a net current that may overwhelm in magnitude the signal that is to be measured, when selection is to be made at a satisfactorily high rate.